1. Field of the Invention
The present invention relates to a wavelength tunable laser module and a wavelength tunable filter.
2. Description of the Related Art
The transmission capacity of optical communication systems has been increasing year by year. In response to such a transmission capacity increase, a wavelength-division multiplexing (WDM) technology is put to practical use as a technology for providing high-speed, large-capacity optical communication at a low cost. The WDM technology makes it possible to simultaneously use many monochromatic light waves (several dozen to one hundred wavelengths) that differ in frequency by 50 GHz or 100 GHz, and transmit different signals at various wavelengths. Further, the use of the WDM technology significantly reduces the cost of optical fiber installation because it increases the transmission capacity of a single fiber by more than several tens of times.
Previously used WDM light sources were such that multiple modules, which were obtained by placing a semiconductor laser element within a housing and connecting its driving semiconductor IC and wiring to it, were required for various wavelengths.
To manufacture a laser, it was necessary to produce a crystal for each wavelength. Further, the module had to be manufactured for each wavelength. This caused a problem of increased cost. To address such a problem, a wavelength tunable module was developed to make it possible to vary the wavelength as desired. As the wavelength tunable module is capable of varying the light wavelength in a range of about 40 nm, it reduces the number of required semiconductor lasers and makes it possible to offer it at a low cost. Therefore, the wavelength tunable module is now used as a major WDM light source.
Various methods were studied for use with a wavelength tunable laser. As an example, a GCSR (Grating assisted Codirectional coupler laser with rear Sampled grating Reflector) will now be described with reference to IEEE Photonics Technology Letters, Vol. 7, No. 7, July 1995, pp. 697-699. FIG. 1 is a cross-sectional view illustrating the structure of a GCSR laser. FIG. 2 is a spectrum illustrating the operating principle of the GCSR laser. This laser is configured so that a reflector with grating structure 12, a phase control region 13, a codirectional coupler 14, and a gain region 15 are optically connected. These regions are delimited in accordance with the arrangement of electrodes 7, 8, 9, and 10.
A striped low-refractive-index waveguide layer 2 is placed in the reflector with grating structure 12, the phase control region 13, and the codirectional coupler 14, which are on an n-type InP substrate 1. A striped gain layer (luminescent layer) 4 is placed in the gain region 15. On top of the low-refractive-index waveguide layer 2, a high-refractive-index waveguide layer 3 is placed in the reflector with grating structure 12, the phase control region 13, and the codirectional coupler 14 via a p-type clad layer. Further, on top of the high-refractive-index waveguide layer 3, via a p-type clad layer, a grating layer 5 is placed in the reflector with grating structure 12, a p-type clad layer is placed in the phase control region 13, and a long-period grating 6 is placed in the codirectional coupler 14. Again, on top of such layers, via a p-type clad layer, the electrodes 7, 8, 9, and 10, which are separated from each other to match the reflector with grating structure 12, the phase control region 13, the codirectional coupler 14, and the gain region 15, respectively, are formed as mentioned earlier. Furthermore, below the n-type InP substrate, a common electrode 11 is placed to cover all the above-mentioned regions (the reflector with grating structure 12, the phase control region 13, the codirectional coupler 14, and the gain region 15).
The low-refractive-index waveguide layer 2 and the high-refractive-index waveguide layer 3 are equal in length and extended into the gain region 15 (to a position that overlaps with an end of the electrode 10). More specifically, the employed structure is such that the start point 18 of the high-refractive-index waveguide layer is the same as the start point 19 of the low-refractive-index waveguide layer, and that the start point 17 of the long-period grating is positioned inside.
As shown in FIG. 2, laser oscillation occurs in a longitudinal mode 33 in the vicinity of a wavelength at which the peak of a transmission spectrum 31 of the codirectional coupler coincides with the peak of a reflection spectrum 32 of the reflector with grating structure. The transmission spectrum 31 of the codirectional coupler and the reflection spectrum 32 of the reflector with grating structure can be adjusted by means of current injection to vary the wavelength of laser oscillation 34.
There is the following problem with the above-described laser. The reflector with grating structure 12, the phase control region 13, and the codirectional coupler 14 can be formed by the same process until a grating formation layer is reached. However, the gain region 15 is an entirely different structure that has to be formed by a different process. The reason is that the gain region 15, which gives priority to luminous efficiency, differs in functionality from an optical waveguide, which gives priority to optical confinement efficiency. As the longitudinal structures completely differ from each other as described above, it is difficult to connect the gain layer 4 to the low-refractive-index waveguide layer 2 with high accuracy. In reality, the n-type InP 1 is 0.9 μm in thickness, the high-refractive-index waveguide layer 3 is 0.34 μm in thickness, and the low-refractive-index waveguide layer 2 is 0.2 μm in thickness. Therefore, if the gain layer 4 is first grown to eliminate an unnecessary portion, and then the n-type InP 1, the high-refractive-index waveguide layer 3, and the low-refractive-index waveguide layer 2 are regrown, the low-refractive-index waveguide layer 2 cannot be smoothly connected to the gain layer 4 due to significant difference in thickness. This is also true in a case where the order of growth is reversed.
A wavelength tunable filter that addresses the above problem is disclosed in Japanese Patent Application Laid-Open Publication No. 2005-327881. FIG. 3 is a bird's-eye view illustrating the structure of the wavelength tunable filter disclosed in Japanese Patent Application Laid-Open Publication No. 2005-327881. The structure shown in FIG. 3 includes a low-refractive-index waveguide layer and a high-refractive-index waveguide layer, which are formed on the surface of a substrate and extended in parallel with each other (this structure is referred to as the lateral codirectional coupler). As shown in FIG. 3, a left-hand waveguide layer 42 and a right-hand waveguide layer 43 are formed on a substrate 41. Further, a left-hand waveguide 44 and a right-hand waveguide 45, in which a long-period grating 46 is formed, are formed in the substrate and positioned in parallel with each other. This wavelength tunable filter functions as a wavelength selection filter because only a particular wavelength λ is allowed to move between the left-hand waveguide layer 42 and right-hand waveguide layer 43. Forming the wavelength selection filter in this manner reduces the level difference between the optical waveguides and makes it possible to establish a smooth connection to a gain region (not shown).
A wavelength tunable laser that addresses the aforementioned problem is disclosed in Japanese Patent Application Laid-Open Publication No. 2000-223774. The wavelength tunable laser disclosed in Japanese Patent Application Laid-Open Publication No. 2000-223774 is configured so that a gain region is sandwiched between a Mach-Zehnder interferometer filter and an SSG-DBR filter.